1. Field of the Invention
This invention relates to a method for fabrication of ceramic thin films, in particular, both dense and porous ceramic thin films, utilizing spray coating. Fabrication of thin (less than or equal to about 10 microns) non-porous (dense) electrolyte layers is a key process in the fabrication of reduced-temperature (less than or equal to about 800.degree. C.) solid oxide fuel cells and other solid state devices. This invention is an economical, scaleable and reliable method for the fabrication of non-porous ceramic films such as electrolyte layers used in solid oxide fuel cells as well as porous ceramic thin films suitable for use in, for example, ultra-filtration applications.
2. Description of Related Art
Practical applications of solid oxide electrochemical devices at temperatures below about 800.degree. C. require a thin film sold electrolyte. Such films are currently produced utilizing a chemical vapor deposition method as taught, for example, by U.S. Pat. No. 5,145,720 to Toshio et al., an electrochemical vapor deposition method as taught by U.S. Pat. No. 4,609,562 to Isenberg et al., and sputter deposition as taught by U.S. Pat. No. 5,656,387 to Barnett et al. Chemical vapor deposition and electrochemical vapor deposition and sputter deposition are limited by long process times and high cost.
A number of additional methods have been used to fabricate oxide thin films at low cost. U.S. Pat. No. 5,085,742 to Dollard et al. teaches a method using thermoplasma spraying of yttria stabilized zirconia on doped lanthanum manganite air electrode tubes to provide an electrolyte layer. However, an additional electrochemical vapor deposition is required in order to fill the pores of the plasma spray coated layer.
U.S. Pat. No. 5,358,735 to Kawasaki et al. teaches a method in which a solid oxide powder is sprayed onto a substrate and then impregnated by a salt solution, thereby filling the pores of the spray coated layer. The thin film is then subjected to a high temperature treatment to improve air tightness of the solid oxide thin film formed on the substrate. The heat treatment temperature is between about 1300-1500.degree. C. dependent upon the composition of the impregnated salt solution.
U.S. Pat. No. 5,585,136 to David et al. teaches a sol-gel process for fabricating solid oxide thin films on a substrate. U.S. Pat. No. 5,368,667 to Minh et al. teaches a calender rolling method and Souza et al., Journal of the Electrochemical Society, Volume 144, No. 3 (1997) reports a method for fabricating yttria stabilized zirconia thin films by dipcoating. The thin films of both of these methods are subjected to a high temperature (1400-1600.degree. C.) sintering for airtightness, but quality control, composition-gradient coatings, surface roughness modification and large coating areas are problematic with these methods.